Plankton is one of the main components in the food chain system in water. In thispaper,the plankton ecosystems studied by formulating a mathematical model in nonlinear ordinary differential equations system, where the growth of plankton influenced by oxygen deficit. This model includes four variables i.e.,concentration of nutrients, density of algae, density of zooplankton and concentration of dissolved oxygen. There are four equilibrium points, obtained. Based on stability analysis conducted, one of the equilibrium points was
stable under certain conditions. Numerical simulations were conducted to convince that the
ecosystem reaches a stable condition.